1. Field of the Invention
The present invention relates to an X-ray fluoroscopic imaging system in which a sensor unit is physically separable, an inter-unit synchronization method of the same, and a computer program.
2. Description of the Related Art
Attempts have been made to improve the handling ability of a sensor unit of an X-ray imaging system by giving a physically separable unit construction to the sensor unit, thereby improving the user-friendliness of the system. X-ray imaging is classified into X-ray fluoroscopic imaging and X-ray still image imaging. In X-ray fluoroscopic imaging, radiography is performed frame by frame.
FIG. 1 shows an example of the configuration of an X-ray imaging system in which a sensor unit is physically separated. FIG. 1 is a view for explaining an example of the X-ray imaging system in which a sensor unit is separated. This X-ray imaging system includes a sensor unit 110, image processing unit 120, and X-ray generation unit 130.
The sensor unit 110 is physically separable from other units.
The image processing unit 120 includes a timing generation unit 121, and performs image processing on a radiographed image transmitted from the sensor unit 110. The timing generation unit 121 generates a trigger signal for performing synchronization control on the sensor unit 110 and X-ray generation unit 130.
The X-ray generation unit 130 includes an X-ray tube 131 and collimator 132. The X-ray generation unit 130 generates X-rays, and exposes the sensor unit 110 to the X-rays. The X-ray tube 131 generates X-rays for the X-ray exposure. The collimator 132 adjusts the X-rays for exposure.
The case where image data is read out from the sensor only once for every X-ray exposure will be explained below with reference to FIG. 2. FIG. 2 is an example of a timing chart of exposure and readout when using a CMOS sensor. X-ray exposure 201 is made active by an exposure trigger 200 transmitted to the X-ray generation unit 130, and readout 203 is started by a readout trigger 202. Since a delay time is necessary from the start of X-ray exposure to the start of readout from the sensor, the readout trigger 202 has a predetermined offset 204 from the exposure trigger. Also, a predetermined time is necessary before readout is completed. Therefore, no accurate image data can be read out if the next X-ray exposure is started before readout is completed. Accordingly, a time 205 from the completion of readout from the sensor to the start of the next X-ray exposure must be positive.
The case where readout is performed twice for every X-ray exposure will be explained below with reference to FIG. 3. FIG. 3 is an example of a timing chart of exposure and readout when using a LANMIT sensor. When using the LANMIT sensor, X-ray intensity distribution information is calculated for each frame by subtracting charge distribution information when no X-rays are applied from that immediately after X-rays are applied. Therefore, readout must be performed twice.
As in the case explained with reference to FIG. 2, a time 300 from the completion of the second readout to the start of the next X-ray exposure must be positive in this case as well.
As explained above, even in the arrangement in which the sensor unit 110 is physically separable as shown in FIG. 1, the X-ray generation unit 130 and sensor unit 110 must operate in strict synchronism with each other. To meet this demand, trigger signal transmission using a dedicated signal line 140 as shown in FIG. 1 is performed in the X-ray imaging system in which the sensor unit 110 is separable.
The merit, however, of the X-ray imaging system in which the sensor unit 110 is separable is that the portability of the sensor unit 110 improves and imaging can be performed by setting the sensor unit 110 in various positions. From this point of view, the sensor unit 110 is desirably completely wireless. To make the sensor unit 110 wireless, how to handle the power supply and large-volume image data is a problem. However, the largest problem is how to wirelessly synchronize the sensor unit 110 and X-ray generation unit 130 as described above.
Examples of protocols normally used in wireless communication are IEEE802.11 (wireless LAN) and IEEE802.15.3 (UWB). In wireless communications using these protocols, data communication can be performed in a constructed network by using a packet containing a header and payload. However, it is difficult to establish synchronization by defining a dedicated packet on a communication protocol of, for example, a wireless LAN. This is so because the necessary arbitration time is uncertain in CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) as a best effort type access control scheme that is a general wireless communication protocol.
On the other hand, a quality priority communication method taking account of Quality of Service (QoS) using the TDMA (Time Division Multiple Access) scheme instead of the CSMA/CA scheme is also defined in a wireless communication protocol. FIG. 4 shows an example of the configuration of a super frame 400 as a communication unit of IEEE802.15.3 as a wireless communication standard using the TDMA scheme. As shown in FIG. 4, this wireless communication standard performs wireless communication by repeating the super frame 400. In the structure of a super frame 400 #m, a beacon 401 #m contains time allocation information of constituent elements (Contention access period, MCTA, and CTA) in the super frame 400 #m.
A node having received the beacon 401 counts the preferential communication enable period (MCTA) allocated to the node in units of μsec by using an internal timer, and performs QoS communication at the designated time. The time required for wireless arbitration can be shortened by thus performing QoS communication.
Even when the arbitration time is unlimitedly shortened, however, it is necessary by analyzing the protocol to extract actual timing information contained in the payload from a wirelessly communicated wireless packet. A delay occurs when using the extracted timing information as a trigger signal. This time loss caused by the protocol analysis is also a factor that makes synchronization by wireless packets difficult.
Japanese Patent Laid-Open No. 2006-305106 describes a method of synchronizing the sensor unit 110 and X-ray generation unit 130 in a wireless environment in which the two units are physically separated. In this method, the separated units each contain an internal timer, the two timers are synchronized through a wired signal line before the two units are separated, and the two units are synchronized on the basis of the values of the internal timers after the units are separated.
There is also an arrangement as shown in FIG. 5. FIG. 5 shows an example of the system configuration of a wireless X-ray imaging system. The same reference numerals as in FIG. 1 denote the same parts in FIG. 5, and a repetitive explanation will be omitted. This configuration includes a second wireless link 501 dedicated for synchronization in addition to a wireless link 500 for data communication. The second wireless link 501 requires no communication arbitration and only a short protocol analyzing time. Trigger signals are transmitted by using the dedicated wireless link 501.
When using the method described in Japanese Patent Laid-Open No. 2006-305106, however, an error occurs between the internal timers of the two units with the elapse of time, and this makes synchronization impossible. In addition, the method using the additional wireless link dedicated for synchronization increases the cost.
Accordingly, the present invention provides a wireless X-ray fluoroscopic imaging system that can be used with a configuration using a single wireless link, and reduces synchronization errors between units.